Christmas 2009 Ice Storm Analysis

THE SITUATION: This will be a highly complex system (aren't they all?) involving a long "front end" period of freezing rain on Christmas Eve Night extending into Christmas Day. Early indications are that areas mosts affected Christmas Eve are likely to be west of a line extending from Carroll County, MD  to Northern and Western Virginia as well as portions of the WV panhandle. Although surface cold air will be marginal (~32 F) across the I-95 corridor, computer model projections and convention wisdom both suggest that low-level cold air will not erode as quickly as anticipated.

A SAMPLE MAP: Shown below is the 7AM computer model projection from today by the GFS for 2 meter temperatures, 10 meter winds and precip by 7PM Christmas Eve. This link will take you to a larger map that's easier to read. A key feature to notice include the 0 C line- which is nearly to Raleigh, NC. That means one thing: Any precip that reaches the ground Thursday evening across the Mid-Atlantic is likely to be sleet or freezing rain, and may last into the early morning hours.

THE EVOLUTION: This NWS forecast map loop provides a quick glance of the timeline. We expect a sprawling 1035 millibar high pressure system, currently near the James Bay, will relocate to southern Ontario by Thursday 12/24. This will be the source region for extensive cold air to funnel south ahead of a  very strong developing surface low in the mid-Mississippi Valley. Moisture feeding northeast ahead of this storm will over-ride cold air at the surface east of the Blue Ridge mountains. Our major concern centers on these indications of "cold air damming" and the potential exists for this to become a dangerous-to-crippling ice event for some during a crucial travel period.

THE PREPARATION:  It would be advisable for homeowners to clear snow away from nearby street storm drains, home downspouts and sump pump outlets. When a big rainstorm follows a snowstorm (as it did in January 1996), the biggest threat to all residents and property is often flooding caused by blocked storm drains. The daily refreezing of snow around storm drains this week, followed by significant icing, followed by heavy rain will create significant urban flooding. The water will have nowhere to go but your basement, parking lot or backyard. School systems need to factor the potential of this storm into their planning decisions regarding re-opening.

THE ACKNOWLEDGEMENTS: Members of our forecast team, the advisory board, and readers in the comments all pegged the potential of this storm starting Sunday afternoon. Special mention goes out to Mr. B of Greencastle, PA; PasadenaMatt, Andy of Southern York County, and Mr. Harris of Glen Burnie, MD. In addition, Snowlover and Winterman were on board in gathering data and observations. All these fine gentlemen were spot on in detection of changes in the computer models that led our team to prepare the current storm statement. We look forward to continued collaboration in the weeks ahead for it is rumored Mother Nature has quite a playbook of acts ready to show.